Igneous rocks form from liquid or molten material called magma (in the ground) or lava (on the surface). It is sometimes believed that the entire interior of the Earth is liquid; this is not the case.Except for the Earth’s outer core and a few minor pockets and zones near the surface where the materials are molten, the Earth is a solid body. Thus, the source for volcanoes and igneous rocks is controlled by geologic processes in the crust and upper mantle. This chapter will describe the classification of igneous rocks, the unique processes that form magmas, types of volcanoes and volcanic processes, volcanic hazards, and igneous landforms.

If a lava is extruded onto the surface (or intruded into shallow fissures near the surface), the resulting igneous rocks are called extrusive (or volcanic). Extrusiveigneous rocks have a fine-grained texture (aphanitic) in which many mineral grains are too small to see with the unaided eye. In addition, quickly cooled extrusive material isn’t a mineral at all but rather is volcanic glass, which doesn’t have a crystalline structure and therefore isn’t a mineral. The fine-grained texture indicates that the lava cooled quickly and minerals didn’t have time to grow visible crystals. In fine-grained aphanitic rocks, mineral crystals can be studied under a petrographic microscope .

Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. The different colors are unique minerals. The black colors are likely two or three different minerals. Source: Peter Davis

Basalt is a classic fine-grained extrusive igneous rock. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine.

In addition to aphanitic and phaneritictexture, some igneous rocks have a few coarse-grained minerals surrounded by a fine-grained material in a texture called porphyritic. The large crystals are called phenocrysts and the fine-grained surrounded material is called the groundmass. This texture indicates a multi-stage cooling history in which the magma body was cooling slowing deeper under the surface and then later rose to shallower depth or extruded at the surface, thus indicating different stages of cooling.

An extreme version of scoria occurs when volatile-rich lava is very quickly quenched and becomes a meringue-like froth of glass called pumice. Some pumice is so full of vesicles (pore space) that it can float.

Welded tuffAs the solid parts of the eruption (called tephra) settle back to earth, they form a rock called pyroclastic, “pyro” referring to the igneous or fire nature of the material and “clastic” referring to rock fragments. Tephra materials are named based on size—ash (<2mm), lapilli (2-64 mm), blocks and bombs (>64mm). Pyroclastictexture is usually recognized by the angular shape of crystals and the presence of shards of glass and rock fragments. Rocks formed from these deposits are called tuff. If it accumulates while hot, crystals may be deformed and the mass may be welded together, forming a welded tuff.

Magma chambers, like this one pictured beneath Yellowstone, can form a pluton.Plutons—Any rock body that formed from a cooled and crystallized magma chamber is called a pluton. A magma chamber is a large reservoir under a volcano that holds a large supply of magma. The term stock can also be used for a pluton. The processes by which rising magma (sometimes known as a diapir) intrudes into overriding and surrounding rock are poorly understood. For example, what happens to the volume of pre-existing rock that is replaced by the pluton? That plutons exist and have intruded into pre-existing country rock is a matter of observation. Was the country rock shouldered aside? Was it consumed within the magma or stoped (i.e. pieces of it broke off and settled through the rising magma)? Is the idea of a magma chamber oversimplified, and really a series of dikes, combining, create a pluton? How plutons are emplaced is a subject of ongoing geological inquiry .

Half Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite.Batholiths—These are large masses of plutons (typically felsic) associated with the roots of mountain belts. The massivegraniteformations of Yosemite National Park and the Sierra Nevada Mountains are batholiths that were emplaced in the core of the Sierra Nevada when they formed millions of years ago and are now exposed by subsequent uplift and erosion. Batholiths and stocks are discordant intrusions, i.e. they cut across and through surrounding country rock. Fluid magmas may also intrude into near surface rocks by taking advantage of specific weaknesses in pre-existing rocks.

Dikes—When magma follows a cross cutting weakness like a crack or fissure, the resulting cross-cutting feature is called a dike. Because of this, dikes are often vertical or diagonal relative to the rock layers that they intersect. Like batholiths, dikes are discordant intrusions. Dikes are important to geologists, not only for the study of igneous rocks themselves but also for dating and interpreting the geologic history of an area. The dike is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time, may be used to assign actual dates to sedimentary sequences. Dikes often can be dated using radioactiveisotopes and help determine the age of sedimentary rocks.

Three intrusive dikes are exposed in this figure at Spanish Peaks, Colorado. The volcanoes have partially eroded away exposing the radial dikes. (Source: G. Thomas)

Igneous sill intruding between Paleozoic strata in Nova ScotiaSills—Magma may exploit a weakness between sedimentary layers by intruding between the layers. shouldering them apart and squeezing in between them. Such an intrusive structure is called a silland is aconcordant intrusion, i.e. it is parallel to the country rock. Sills are also important for the study of geologic history for they are younger than the sediment both below and above the intrusion. As seen above, radiometric dating of these igneous intrusions is an important method of relative dating sedimentary rocks.

Laccolith forms as a blister in between sedimentary strata.Laccoliths—These are blister-like intrusions of magma between sedimentary layers (i.e.concordant). A famous example of a topographic feature formed by this process is the Henry Mountains of Utah. Laccoliths typically bulge upwards, while a downward-bulging and similar intrusion is called a lopolith.

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4.2 Bowen’s Reaction Series

Normal L. Bowen

Norman L. Bowen (1887-1956) was an early 20th Century geologist who exemplifies application of the scientific method. Bowen studied igneous rocks and noticed that in igneous rocks, certain minerals always occur together and these mineral assemblages exclude other minerals. Curious as to why, and with the hypothesis in mind that it had to do with the temperature at which the rocks cooled, he set about conducting experiments on igneous rocks in the early 1900s. In those experiments, he ground rocks and combinations of rocks into powder, put the powder into metal capsules and sealed them. He heated them to various temperatures and then quenched the capsules. After opening the capsules, he cut thin slices (called thin sections) of the contents and studied the minerals present.

Bowen working with specimens at his petrographic microscope

When he opened the quenched capsules, he found a glass surrounding mineral crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a period of several years, showed that the common igneousminerals crystallize from magma at different temperatures and that minerals occur together in rocks with others that crystallize within similar temperature ranges. Bowen’s work laid the foundation for understanding igneouspetrology (the study of rocks) and resulted in his book, The Evolution of the Igneous Rocks in 1928 .

Bowen’s Reaction Series. Higher temperature minerals shown at top (olivine) and lower temperature minerals shown at bottom (quartz). (Source Colivine, modified from Bowen, 1922)Olivine, the first mineral to crystallize in a melt.Bowen’s Reaction Series (shown above) describes the temperature at which minerals crystallize when cooling or melt when heated. The temperature scale ranges from around 700˚C (at the low end where all minerals have crystallized into solid rock) to around 1250˚C (at the upper end where all minerals have melted) . Since Bowen conducted his experiments at the pressure of the Earth’s surface, the temperature of crystallization would be different deeper in the Earth where pressures are much higher (discussed in the next section).

Citrine, a variety of quartz showing conchoidal fractureThe compositions on the right side of the diagram, felsic, intermediate, mafic, and ultramafic, refer to the silica composition of each group. The arrows on the right show increasing abundance of key ions. Think in terms of the energy regimes at which these elements form bonds with other elements and the heat energy represented by atomic motion at the various temperatures. Energy is involved in the forming and breaking of bonds between ions and bonds form when the energy (temperature) is right for the ions involved.

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4.3 Magma Generation

Magma and lava contains three components – melt, solids, and volatiles. The liquid part, called melt, is made of ions from minerals that have already melted. The solid part, called solids, are crystals of minerals that have not melted yet and are floating in the melt. Volatiles are gaseous components dissolved in the magma such as water vapor, carbon dioxide, sulfur, and chlorine . The presence and amount of these three components affect the physical behavior of the magma. This will be considered later in the chapter.

4.3.1 Geothermal Gradient

Geothermal gradient

Although it is very hot under the Earth’s surface, the crust and mantle are mostly solid. This heat inside the Earth is caused by residual heat left over from the original formation of Earth and from radioactive decay. The rate at which temperature increases with depth is the geothermal gradient. The average geothermal gradient in the upper 100 kilometers of the crust is generally about 25°C per kilometer (km). So, for every kilometer of depth, the temperature increases by 25 °C.

Pressure-temperature diagram showing temperature in degrees Celsius on x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and green solidus line represents at temperature and pressure regime at which melting begins. Rocks at pressures and temperatures left of the green line are solid. If pressure or temperature conditions change so that rocks pass right of the green line, then they will start to melt. (Source: Woudloper)Pressure-temperature diagrams illustrate the geothermal gradient and the behavior of rock by graphing depth (pressure) and temperature (see figure). The figure shows the geothermal gradient changing with depth through the the crust and mantle. The diagram shows geothermal gradient as a red line and at 100 km the temperature is about 1,200°C. In addition, the pressure at bottom of the crust (shown here as depth; 35 km deep) is about 10,000 bars . Bar is a measure of pressure, 1 bar being normal atmospheric pressure at sea-level. At these pressures and temperatures in the Earth, the crust and mantle rocks are solid. On the P-T diagram, the green solidus line shows the pressures and temperatures at which rocks start to melt. Since the geothermal gradient (red line) is always left of the solidus (green line) to a depth of 150 km, then the rocks are solid. The solidus line slopes to the right because the melting temperature of any substance depends on the pressure. A higher pressure at greater depth requires higher temperature to melt rock. In another example, water boils at 100°C at an atmospheric pressure close to 1 bar. But if the pressure is lowered, as shown on the video below, then water boils at a much lower temperature.

Four P-T diagrams show temperature in degrees Celsius on x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and green solidus line represents at temperature and pressure regime at which melting begins. Each of the four P-T diagrams are associated a tectonic setting as shown by a side-view (cross-section) of the lithosphere and mantle.

4.3.2 Decompression Melting

Progression from rift to mid-ocean ridge, the divergent boundary types. Note the rising material in the center.Magma is created at the mid-ocean ridge by decompression melting. The mantle is solid but is flowing under great pressure and temperatures due to convection. Rock is not a good conductor of heat so as mantle rock rises, pressure is reduced along with the melting point (the green line) but the rock temperature remains about the same and the rising rock begins to melt. Pressure changes instantaneously as the rock rises but temperature changes slowly because of the low heat conductivity of rock. On the figure above, setting B: mid-ocean ridge shows a mass of mantle rock at a pressure-temperature location X on the P-T diagram as well as its geographical location on the cross section under a mid-ocean ridge. At this location, the P-T diagram shows the red arrow increasing to the right. Thus, hotter rock is now shallower, at a lower pressure, and the new geothermal gradient (red line) shifts past the solidus (green line) and melting starts. As this magma continues to rise at divergent boundaries and encounters seawater, it cools and crystallizes to form new lithospheric crust.

Early in Earth history when the continents were forming, less dense and more silica-rich magmas rose to the surface and solidified into silica-rich granitic continents. Today, the old granitic cores of the continents are shown below in orange as the shields. The next section describes how these silica-rich magmas evolve from ultramafic magmas.

Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents.

Schematic diagram illustrating fractional crystallization. If magma at composition A is ultramafic, as the magma cools it changes composition as different minerals crystallize from the melt and settle to the bottom of the magma chamber. In section 1, olivine crystallizes; section 2: olivine and pyroxene crystallize; section 3: pyroxene and plagioclase crystallize; and section 4: plagioclase crystallizes. The crystals are separated from the melt and the remaining magma (composition B) is more silica-rich. (Source: Woudloper)

What does an underwater volcanic eruption look like? Basaltic magma erupts underwater forming pillow basalts and/or in small explosive eruptions. In association with these seafloor eruptions, an entire underwater ecosystem thrives in parts of the mid-ocean ridge. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents (also known as black smokers).

Distribution of hydrothermal vent fields.This hot water, up to 380 °C (716 °F), is heated by the magma and dissolves many elements that supports the ecosystem. Deep underwater where the sun cannot reach, this ecosystem of organisms depends on the heat of the vent for energy and vent chemicals as its foundation of life called chemosynthesis. The foundation of the ecosystem is hydrogen sulfide-oxidizing bacteria that live symbiotically with the larger organisms. Hydrogen sulfide (H2S – the gas that smells like rotten eggs) needed by these bacteria is contained in the volcanic gases emitted from the hydrothermal vents. The source of most of this sulfur and other elements is the Earth’s interior . Below are three short videos on a deep-sea submersible submarine and deep-sea hydrothermal vents.

Volcanoes at Subduction Zones

Distribution of volcanoes on the planet. Click here for an interactive map of volcano distributions.

4.5.2 Volcano Features and Types

Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other features. The following are the main components of a volcano:A conduit is the narrow pipe that connects the magma chamber to the surface at the ventwhere lava erupts. A large depression at the top that is called a crater, and the largest craters are called caldera, such as the Crater Lake Caldera in Oregon. A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber. Many volcanic features are derived from one basic measure of a lava: viscosity. Viscosity is the resistance to flowing by a fluid. Counterintuitively, a high viscosity means a gooey, less able to flow magma, similar to toothpaste. A low viscosity magma will flow more easily, like the volcanism that occurs in Hawaii on shieldvolcanoes.

Shield Volcano

Kilauea in Hawai’i.The largest volcano is a shield volcanoand is characterized by broad, low-angle flanks, a small vent or groups of vents at the top, and basaltic magma. The name “shield” comes from the side view resembling a medieval warrior’s shield. They form slowly from many low-viscosity basaltic lava flows that can travel long distances, hence, making the low-angle flanks. Because the magma is basaltic and low viscosity, the eruption style is not explosive but rather effusive, meaning that volcanic eruptions are small, localized, and predictable. Therefore, this eruption styles isn’t much of a hazard. Mauna Loa (info) and the more active Kilauea (info) in Hawai’i are good examples. Shieldvolcanoes are also found at Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift. The largest volcanic edifice in the Solar System is Olympus Mons on Mars. This is a shield cone as large as the state of Arizona indicating little if any platetectonic activity on Mars as the volcano erupted over the same hotspot for millions of years .

In basaltic lava flows, the low viscosity lava can easily flow, and it tends to harden on the outside but continue to flow internally within a tube. Once the interior flowing lava subsides, the tube can be left as an empty lava tube. Lava tubes famously make caves (with or without collapsed roofs) in Hawai’i, Northern California, the Columbia RiverBasalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho. Fissures, cracks that originate from shield-style eruptions, are also common. Magmas from fissures are typically very fluid and mafic. Some fissures are caused by the volcanic activity itself, and some can be influenced by tectonics, such as the common fissures parallel to the divergent boundary in Iceland.

Devils Tower in Wyoming has columnar jointing.

Since basalt flows are thick accumulations of lava with a homogenous composition that flows quickly, when the lava begins to cool it can contract into columns with a hexagonal cross section called columnar jointing. This feature is common in basaltic lava flows but can be found in more felsic lavas and tuffs as well.

Stratovolcano

Mount Rainier towers over Tacoma, Washington.

A stratovolcano, sometimes called composite cones, have steep flanks, symmetrical cone shapes, distinct craters, and rise prominently above the surrounding landscape. Examples include Mount Rainier in the Cascade Range in Washington and Mount Fuji in Japan. Stratovolcanoes can have magma with felsic to maficcomposition. However, felsic to intermediate magmas are most common. The term “composite” refers to the alternating deposition of pyroclastic materials (like ash) and lava flows. The viscous nature of the more common intermediate and felsicmagma results in steep flanks and explosive eruption styles. Stratovolcanoes are made of lava flows and ash.

Lava Domes

Lava domes have started the rebuilding process at Mount St. Helens, Washington.

Lavadomes are relatively small accumulation of silica-rich volcanic rocks, such as rhyolite and obsidian, that are too viscous to flow, and therefore, pile high close to the vent. The domes often form within the collapsed crater of a stratovolcano near the vent and grow by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. A good example of a lava dome is inside of a collapsed stratovolcano crater is Mount Saint Helens. Examples of a stand alone lava dome are Chaiten in Chile and the Mammoth Mountain in California .

Caldera

Timeline of events at Mount Mazama.Wizard Island sits in the caldera at Crater Lake.

Calderas are usually large, steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. Calderas are generally very large with a diameter up to 15 miles. Although the word caldera only refers to the vent, many use caldera as a volcano type, typically formed by high-viscosity felsicvolcanism with high volatile content. Crater Lake, Yellowstone, and Long Valley Caldera are good examples. At Crater Lake National Park in Oregon, about 6,800 years ago Mount Mazama was a composite volcano that erupted in a large explosive blast ejecting large amounts of volcanicash. The eruption rapidly drained the underlying magma chamber causing the top to collapse forming a large depression that later filled with water. Today a resurgent dome is found rising up through the lake as cinder cone, called Wizard Island .

Map of calderas and related rocks around Yellowstone.

The Yellowstone caldera erupted three times in the recent past, at 2.1, 1.3, and 0.64 million years ago. Each eruption created large rhyolite flows and pyroclastic flows of ash that solidified into tuff. These extra large eruptions rapidly emptied the magma chamber causing the roof to collapse and form a caldera. Three calderas are still preserved from these eruptions and most of roads and hotels of Yellowstone National Park are located within the caldera. Two resurgent domes are located within the last caldera.

The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.Yellowstone volcanism started as a hot spot under the North American lithosphere about 17-million years ago near the Oregon/Nevada border. As the North American plate slid over the stationary hotspot, surface volcanism moved through and helped form Idaho’s Snake River Plain, eventually moving to its current location in northwestern Wyoming. As the plate move over the stationary hotspot to the southwest, it left a track of past volcanic activities .

Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall from Mt. St. Helens (yellow).

The Long Valley Caldera near Mammoth California is a large explosive volcano that erupted 760,000 years ago and dumped a large amount of ash throughout the United States, similar to the Yellowstone Eruptions. This ash formed the large Bishop Tuff deposit. Like the Yellowstone caldera, the Long Valley Caldera contains the town of Mammoth Lakes, a major ski resort, an airport, and a major highway. Further, there is a resurgent dome in the middle and active hot springs .

Cinder Cone

Sunset Crater, Arizona is a cinder cone.

Cinder cones are small volcanoes with steep sides, made of cindersand volcanicbombs ejected from a pronounced central vent. Typically, they come from mafic lavas with high volatile content. Cinders form when hot lava is ejected into the air, cooling and solidifying before they reach the flank of the volcano. The largest cinders are called volcanicbombs. Cinder cones form in single eruption events that are short-lived and relatively common throughout the western United States .

Soon after the birth of Parícutin in 1943.

A relatively recent and striking example of a short-lived cinder cone is the 1943 eruption near the village of Parícutin, Mexico . The cinder cone started with an explosive eruption shooting cinders out of a vent in the middle of a farmer’s field. Quickly, volcanism continued building the cone to a height of over 300 feet in a week and 1,200 feet in the first 8 months. After the initial explosive gases and cinders were released, growing the cone, basaltic lava poured out around the cone. This order of events is common for cinder cones: first violent eruption, then formation of cone and crater, followed by a low-viscosity lava flow. The Parícutin cinder cone was built over nine years and covered about 100-square miles with ashes, and destroyed the town of San Juan .

Flood Basalts

Map of global flood basalts. Note the largest is the Siberian Traps.

A rare and still unobserved volcanic eruption type are flood basalts. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption is still up for debate. Some famous examples include the Columbia RiverFlood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been responsible for Earth’s largest mass extinction at the end of the Permian (see chapter 8).

Carbonatites

Crater of Ol Doinyo Lengai in 2011. Note the white carbonatite in the walls of the crater.

Arguably the most unusual volcanic activity produces carbonatites. These are a product of carbonate-based volcanism, instead of all other volcanism which is silicate based. Carbonatites only erupt from one volcano on Earth today: Ol Doinyo Lengai, in Tanzania. The lavas are incredibly low viscosity, relatively cold (for lava), black when erupted, but solidify to a brown/grey and eventually to white. These rocks are found occasionally in the geologic record, and are mostly associated with rifting.

4.5.3 Volcanic Hazards and Monitoring

General diagram of volcanic hazards.

Volcanic hazards have been famous for centuries, but recent eruptions are more well documented. The most obvious hazard is the lava itself found within a lava flow, but the hazards posed by volcanoes go far beyond a lava flow. For example, on May 18, 1980, Mount Saint Helens erupted explosively with a giant landslide that removed the upper 1,300 feet (400 m) of the mountain. This landslide was immediately followed by the lateral blast , and covered 230 square miles of forest with ash and debris. The effects of the blast are shown on the before and after images (Figure and Figure). The pyroclastic flow moved at speeds of 50 – 80 miles per hour (80-130 km/hr), flattened trees and ejected a large ash cloud into the air. Watch the 7-minute USGS video for an account of May 18, 1980 that killed 57 .

Human remains from the 79 CE eruption of Vesuvius.

In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered as an archeological site in the 18th century . Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumicelapilli, and collapsed roofs .

Mount St. Helens, the day before the May 18th, 1980 eruption.

Picture 4 months after the major eruption of Mount St. Helens.

Series of still images of the May 18, 1980, eruption of Mt. Saint Helens, Washington showing largest recorded landslide in history and subsequent eruption and pyroclastic flow (By The Associated Press via The Atlantic)

Pyroclastic flows

The material coming down from the eruption column is a pyroclastic flow.

The most dangerous volcanic hazard are pyroclastic flows (video). These flows are from collapses in the eruption column that run downhill. These are mainly explosive eruptions that rapidly eject a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300 ℉. The turbulent cloud of ash and gases races down the steep flanks at high speeds (>120 mph) in the valleys of composite volcanoes. Most explosive, silica-rich, high viscosity magmavolcanoes such as composite cones usually have pyroclastic flows. The rock tuffand welded tuff is the solid form of ash from these pyroclastic flows.

The remains of St. Pierre.

There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanicbombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanicbombs. Two short videos below document eye-witness video of pyroclastic flows. In early 1990’s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this famous short video killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people .

Landslides and Landslide-Generated Tsunamis

Sequence of events for Mount St. Helens, May 18th, 1980. Note that an earthquake caused a landslide, which caused the “uncorking” of the mountain and started the eruption.

The flanks of a volcano are steep and unstable which can lead to slope failure and generate dangerous landslides. For example, the landslide at Mount St. Helens 1980 released a huge amount of materials as the entire north flank collapsed. The landslide moved at speeds of 100-180 mph. These landslide can be triggered by movement of magma, explosive eruptions, large earthquakes, and heavy rainfall. In unique situations, the landslide material can reach water and cause a tsunami. In 1792 in Japan, Mount Unzen erupted causing a large landslide that reached the Ariaka Sea and made a tsunami that killed 15,000 people on opposite shore (info) .

Lahars

Mud line shows the extent of lahars around Mount St. Helens.

A laharis an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris moving down the flanks of a volcano (or other nearby mountains covered with freshly-erupted ash) and entering adjacent river valleys. They form from the rapid melting of snow or glaciers on volcanoes. They are similar to a slurry of concrete but can flow up to 50 mph while still on the steep flanks. Since lahars are slurry-like they can travel long distances in river valleys almost like a flash flood.

Old lahars around Tacoma, Washington.

During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Prehistoric lahar flows have been mapped at major volcanoes such as Mount Rainier near Tacoma, Washington (Rosi et. al. 1999). Prehistoric lahars are occupy riverfloodplain where large cities are located today as shown on the map. Similarly Mount Baker poses a hazard as shown by this hazards map for Mount Baker north of Seattle, Washington . A recent scenario played out when a lahar from the volcano Nevado del Ruiz in Colombia buried a town in 1985 and killed an estimated 25,000 people.

Tephra and Ash

Aman sweeps ash from an eruption of Kelud, Indonesia.Volcanoes, especially composite volcanoes, eject large amounts of tephra(ejected rock materials) and ash(fragments less than 0.08 inches [2 mm]). Tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier. Ash is fine and can be carried long distances away from the vent, and can cause building collapses and respiratory issues like silicosis. Hot ash can be dangerous to those close to the eruption and disrupt services such as airline transportation farther away . For example, in 2010 the Eyjafjallajökull volcano in Iceland created a large ash cloud in the upper atmosphere that caused the largest air travel disruption in northern Europe since a seven-day airline shut down during World War II. No one was hurt but the cost to the world economy was estimated to be billions of dollars .

Volcanic Gases

This cow was a victim of carbon dioxide asphyxiation from Lake Nyos in 1986.

Magma contains dissolved gases. As rising magma reaches the surface the confining pressure decreases allowing gases to escape. This is similar to gases coming out of solution after opening a soda bottle. Therefore, volcanoes when not erupting release hazardous gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S) and hydrogen halides (HF, HCl, or HBr). Carbon dioxide can sink and accumulate in low lying depressions on the earth’s surface. For example, Mammoth Mountain Ski Resort in Mammoth Lakes, California is located within the Long Valley Caldera. Therefore the whole ski resort and town are within the caldera. In 2006, three ski patrol members were killed after skiing into snow depressions near fumaroles that had filled with carbon dioxide (info). Therefore, in volcanic areas where carbon dioxide emissions occur, avoid low-lying areas that may trap carbon dioxide . In rare cases, a volcano can suddenly release gases without warning. Called a limnic eruption, this commonly occurs in crater lakes as gases pour from the water. It infamously occurred in 1986 in Lake Nyos, Cameroon, killing almost 2,000 people due to carbon dioxide asphyxiation.

Volcanic Monitoring

Four main types of seismograms. Note harmonic tremor at the bottom.

Volcano monitoring requires geologist to use many instruments to detect changes that may indicate an eruption is imminent . Some of the main observations include regular monitoring for: earthquakes (including special vibrational earthquakes called harmonic tremor, caused by magma movement), changes in the orientation and elevation of the land surface, and increases gas emission. Very short videos (below) summarize how an increased frequency of earthquakes can show that magma is moving and that an eruption may occur soon. Another video (below) shows how gas monitoring is used to monitor volcanoes and predict eruption. As the magma gets closer to the surface, the gases come out of the magma. A rapid increase of gas emission can indicate an eruption is imminent. This last video (below) shows how a GPS unit and tiltmeter can detect movement of the land indicating that the magma is moving underneath.